In conventional medical diagnostic imaging, the object is to obtain an image of a patient's internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5% or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element.
The need to increase the diagnostic capabilities of radiographic imaging assemblies (film and screen) while minimizing patient exposure to X-radiation has presented a significant, long-standing challenge in the construction of both radiographic films and intensifying screens. In constructing radiographic intensifying screens, the desire is to achieve the maximum longer wavelength electromagnetic radiation emission possible for a given level of X-radiation exposure (that is realized as maximum imaging speed) while obtaining the highest achievable level of image definition (that is, sharpness or resolution). Since maximum speed and maximum sharpness in the screens are not compatible features, most commercial screens represent the best attainable compromise for their intended use.
Examples of radiographic element constructions for medical diagnostic purposes are provided by U.S. Pat. No. 4,425,425 (Abbott et al.) and U.S. Pat. No. 4,425,426 (Abbott et al.), U.S. Pat. No. 4,414,310 (Dickerson), U.S. Pat. No. 4,803,150 (Dickerson et al.), U.S. Pat. No. 4,900,652 (Dickerson et al.), U.S. Pat. No. 5,252,442 (Tsaur et al.), and Research Disclosure, Vol. 184, August 1979, Item 18431.
Conventional supports for intensifying screens include plastic films such as cellulose ester, polyester, polyolefin, and polystyrene films that can be loaded with absorbing or reflective dyes or pigments as desired.
The choice of a support for the intensifying screens (upon which the phosphor layer is disposed) illustrates the mutually exclusive choices that are considered in screen optimization. It is generally recognized that supports have a high level of absorption of emitted longer wavelength electromagnetic radiation produce the sharpest radiographic images. The screens that produce the sharpest images are commonly constructed with black supports or polymeric supports loaded with carbon black. In these constructions, sharpness is improved at the expense of photographic speed because a portion of the otherwise available, emitted longer wavelength radiation is not directed to the adjacent radiographic film.
However, even the best reflective supports known in the art have degraded image sharpness in relation to imaging speed so as to restrict their use to situations wherein image sharpness is less demanding. Further, many types of reflective supports that have been found suitable for other purposes have been tried and rejected for use in screens. For example, the loading of the supports with optical brighteners, widely used as “whiteners”, such as barium sulfate and titanium dioxide has been found incompatible with achieving satisfactory image sharpness with screens.
There exists in the art a class of reflective supports (known as “stretch cavitation microvoided” supports) that are composed of stretched polymeric films having small voids that may contain various particles such as polymeric microbeads. By biaxially stretching the support, stretch cavitation microvoids are introduced into the polymeric films, rendering the films opaque.
Such stretch cavitation microvoided supports have been used in photographic elements, bottles, tubes, fibers, and rods among other articles.
U.S. Pat. No. 4,912,333 (Roberts et al.) describes the use of stretch cavitation microvoided supports composed of a continuous polymeric phase, immiscible microbeads dispersed therein, and reflective microvoids (also called “lenslets”) for fluorescent intensifying screens. The microbeads are composed of polymeric materials with specific refractive indices. Cellulose acetate microbeads are particularly useful.
Copending and commonly assigned U.S. Ser. No. 10/706,524 (filed Nov. 12, 2003 by Laney and Steklenski) relates to phosphor screens having highly reflective microvoided polyester supports.
Problem to be Solved
While various support materials known in the art have been used in commercial products, there remains a need for additional fluorescent intensifying screens that have increased reflectance over the typical radiation range, but particularly in the “near UV” region (typically from about 350 to about 400 mm) of the electromagnetic spectrum. There is a need for such screens that provide increased photographic speed without a significant loss in image sharpness.